thanks once more for the latest fixes to tex-fold-mode. As I will reply in a different thread, they work great, once the scrolling issues are addressed somewhere else.
Nevertheless, some improvement is possible and I am attaching a change that sets mathbf at the same level as the texbf macro with respect to folding and font-lock-mode.
I hope it will be considered for the master branch of AUCTeX..
patch-mathbf.el